In the process of production of steel plate, the steel plate surface is cleaned for various purposes. For example, cleaning of the steel plate before plating or coating, removal of oxide scale (descaling) by pickling hot rolled steel plate, etc. may be mentioned. Regarding descaling, usually steel plate is formed with oxide scale on the steel plate surface in the process of being heat treated and rolled, so the oxide scale has to be removed. That is, the oxide scale is often caught at the rolling rolls and causes damage to the surface of the steel plate at the time of the later step of cold rolling, so descaling is a necessary and essential step. For conventional oxide scale removal, the steel plate is often dipped into a plurality of acidic solutions and continuously run so as to remove the scale by pickling.
Acceleration or increasing efficiency of cleaning of such steel plate, improvement of the cleaning ability, etc. largely depend on the design of the cleaning solution, but as one method for further assisting cleaning at the time of cleaning, the method of applying 20 to 100 kHz ultrasonic waves is described in PLT's 1, 2, and 3. If applying ultrasonic waves inside the cleaning solution, a cavitation phenomenon occurs at the surface of the steel plate being cleaned and the cleaning effect is promoted. That is, due to the ultrasonic waves, the pressure locally falls and becomes lower than steam pressure inside the cleaning solution, so water vapor is formed and the dissolved gas expands resulting in small bubbles or cavities being rapidly formed and quickly collapsing. Due to this, the chemical reaction of the cleaning is promoted and also an impact force is given so the cleaning effect is promoted. Therefore, application of ultrasonic waves is also effective for pickling descaling of hot rolled steel plate.
Further, in PLT 4 or 5, solid particles are made to disperse through the cleaning solution whereby the effect of application of ultrasonic waves is further assisted.
Further, PLT 6 describes addition of microbubbles so as to further improve the cleaning effect due to application of ultrasonic waves. When only applying ultrasonic waves to the cleaning solution and pickling solution, when jointly using microbubbles, the range of propagation of the ultrasonic waves spreads three-dimensionally, so the object being cleaned can be uniformly cleaned.
Further, while the object being cleaned is a glass sheet or semiconductor wafer, PLT 7 discloses feeding a cleaning solution which contains microbubbles to the object being cleaned and applying ultrasonic waves combining a plurality of frequencies. The reason for combining a plurality of frequencies is to crush the microbubbles by the 5 to 800 kHz low frequency ultrasonic waves so as to generate microbubble radicals and to effectively mix the microbubble radicals by the 1 MHz or higher high frequency ultrasonic waves. Effective cleaning becomes possible because of this.
For the pickling descaling, sulfuric acid, hydrochloric acid, nitric acid, fluoric acid, etc. alone or variously mixed to form a pickling solution may be used. To increase the pickling rate of the pickling solution, increasing the acid concentration, raising the pickling temperature, etc. have been attempted, but there are minus aspects such as the increase in chemicals and energy costs and the roughening of the steel material surface after pickling, so there are limits to improvement of the pickling rate and therefore ultrasonic waves are jointly used. However, reduction of the manufacturing costs of steel plate and improvement of the quality of steel plate are desired. For the cleaning and descaling of steel plate as well, further improvement of the cleaning efficiency and improvement of the cleanliness of the surface of steel plate are necessary.